1. Field of the Invention
The present invention relates to an improved actuator for a print wire of a dot matrix printer, and, more specifically, to a plurality of such actuators conveniently associated together in an improved print head for a dot matrix printer. The present invention constitutes an improvement of the dot impact printer and actuator described in U.S. Pat. No. 4,236,836, issued Dec. 2, 1980 in the name of Hodne, and is particularly intended to provide a print head for a dot matrix printer having the capability of operating eighteen, twenty-four or more print wires in a dense pattern which can print high quality, easily readable printed characters, with descenders and ascenders, without the need for the print head to be moved in multiple passes over a printing medium.
1. Brief Description of the Prior Art
Dot matrix printers and print heads therefor are well known in the prior art. Typically, such printers include a plurality of elongated print wires, eachof which is longitudinally, ballistically movable to a print position whereat an end thereof is impacted against an inked ribbon held adjacent to a print medium, such as a sheet of paper, to print a dot thereon. The wires are selectively moved in various combinations so that legible indicia (characters, such as letters, numbers and symbols) made up of patterns of the dots are printed on the paper. In typical prior art matrix printers, each wire is selectively movable between a non-print position and the print position by an armature which is pivoted in response to selective energization of an electromagnet from a pole face of which the armature is normally separated by an air gap. Each armature bears against the end of its associated wire which is opposite the end which effects the printing. The print wires and their actuators--the armatures and the electromagnets--are associated together in an array in the print head which is movable back and forth across the paper as the wires are selectively moved. As is well known, the printing of high quality characters may require multiple passes of the print head across a single "line" of the paper. In order to eliminate such multiple passes of the print head, the number of wires in the matrix print head must be increased. See, for example, "Wire-Matrix Advances Fuel Rise of Impact Printers" in the Aug. 16, 1980 issue of Electronic Design and "18-Wire Print Head--Fast and Good Enough to be Multipurpose" on page 35 of the Sept. 27, 1980 issue of Electronic Design.
The distance each armature must pivot or travel is critical to the performance of the print head, as this distance affects the speed of operation of the print head, the printing energy of the print wires, and the electrical power requirements of the electromagnets. Armature travel must, therefore, be accurately controlled and must be uniform for each actuator in the print head. Excessive variation of armature travel due to cumulative manufacturing tolerances is a well known problem in the prior art. To date, it has, accordingly, been necessary to provide facilities for adjusting the limit of the travel of each armature in the print head. Such adjustment has typically been effected by causing armature to abut a back-stop in its nonprint position. The back-stop may include an adjustable set screw or a piece of resilient material, the position or thickness of which is adjusted for each armature to appropriately set its non-print position. The provision of such back-stops is both complex and costly, and attempts have been made to simplify or eliminate them. See, for example, U.S. Pat. No. 4,222,674 where strict control of critical dimensions is said to eliminate the need for adjustable armature back-stops. Nevertheless, variations of the gaps between each armature and the pole face of its electromagnet may be rather large and likely to have an adverse effect on the uniform performance of each actuator of the print head. Thus, one object of the present invention is the provision of an improved matrix print head in which precise and uniform travel of each armature is achieved without the need for back-stop adjustment.
Back-stops usually include a resilient rubber or plastic material which tends to damp and minimize the rebound of the armatures as they impact thereagainst following pivoting to their print positions. The back-stops of the prior art have the disadvantage that the area of contact between them and the armatures is relatively small, as is the case where the back-stop is a rubber pad on the end of an adjustable set screw. In this event, the deflection of the back-stop upon impact of an armature thereagainst is relatively large, and, consequently, a large rebound of the armature may occur. Armature rebound is a problem in that it leads to erratic armature motion, reduced speed of armature operation, and inability to rapidly print. Thus, another object of the present invention is an improved print head in which the armatures impact against a back-stop over a large area, thereby reducing the deflection of the back-stop to significantly reduce the rebound of the armatures, leading to improved performance.
The head portion of the dot printer disclosed in the '674 patent includes a plurality of actuators for selectively moving a plurality of wires to produce printed characters. In theory, the print head could be constructed to actuate eighteen, twenty-four or a greater number of print wires. Nevertheless, the structure of the print head disclosed in the '674 patent is quite complicated in structure and rather expensive to manufacture. It should be noted that the '674 patent discloses a cup-shaped yoke which mounts a plurality of iron cores. The cores are mounted to the bottom of the cup-shaped yoke in a circular array. Each core is surrounded by a bobbin, around each of which is wound an electric coil. Armatures of the print head are each pivoted on a rather complicated structure formed integrally with the bobbin and are attracted toward the print position by the yoke and the cores when the coils are energized. The present invention relates, in part, to an improvement of and a simplification in the yoke-core structure of the '674 patent, in order to more effectively move the armatures and to improve and simplify the structure of the bobbins, while simplifying, and decreasing the cost of, the assembly and operation of the print head.
The above-referenced '836 patent describes an improved actuator for a dot matrix printer. Theoretically, any desired number of such actuators could be associated in a print head to selectively actuate eighteen, twenty-four or more wires to produce high quality print. However, as the patent is spcifically concerned with improvements in the actuator itself, and not with techniques of conveniently assemblying together a number of actuators, no specific means for conveniently and inexpensively associating a number of actuators together in a print head is disclosed therein.
U.S. Pat. No. 4,230,412 relates to a matrix print head assembly. In this patent, armatures are centrally supported between their ends on an inner pole piece. In order to pivot the armatures to actuate their print wires, an electronmagnetic coil surrounding an outer pole piece (which is adjacent an outer end of each armature) is actuated to attract such outer end. This attraction pivots each armature on its inner pole piece, pivoting its inner end to longitudinally move its associated print wire. Each armature is maintained in a normal, non-print position by an O-ring maintained in a groove which is pushed against the armature between its pivoting point on the inner pole piece and its point of engagement with the print wire. The present invention relates to an improvement of this structure, wherein the armature is more efficiently and simply held and maintained in its non-print position and wherein construction of the print head is rendered more convenient.
U.S. Pat. No. 2,882,368 relates to an electromagnetic relay. Certain principles of this patent are preferably incorporated into the actuator of the present invention. The '368 patent relates to increasing and more efficiently utilizing the force exerted on an armature by appropriate configuration of pole faces and a core associated with the armature, as hereinafter described.
Similarly, the present invention constitutes an improvement and simplification of the wire matrix print head depicted in U.S. Pat. No. 4,279,521.
Prior art dox matrix printers are generally known in the prior art for their simplicity of design, their reliability, their high speed of operation, and their low cost. A major limitaion has, however, been the inferior print quality, stemming from the relatively small number of dots making up the printed characters. Print quality is directly related to the number of print wires used in the print head, which typically has been limited to seven or nine. A principal object of the present invention is to obtain a significant improvement of the print quality achievable by a matrix print head by conveniently and inexpensively doubling or tripling the number of print wires therein. Further, by reducing the diameter of the print wires so the printed characters can be constructed from a larger number and a higher density of small dots, the print quality can be improved to approach the high quality of a typewriter. The high dot density will improve the graphics capability of the printer, provide more freedom and flexiblity in the design of character fonts, and generally help to realize the full potential of matrix printing technology.
As the number of print wires in the matrix print head increases, assembly of the print head becomes more difficult. It is desirable to hold to a minimum and to simplify the number of steps of such assembly so as to minimize the ultimate cost of the print head. It is also desirable to minimize critical adjustments in, and critical assembly operations of, a multiwire print head. These are additional objects of the present invention.